ASoC: Add ssm2518 support

This patch adds a ASoC CODEC driver for the SSM2516. The SSM2516 is a stereo
Class-D audio amplifier with an I2S interface for audio in and a built-in
dynamic range control processor.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>